The advent of digital recording techniques has enabled digital copies of digital content carried by recording media such as optical discs (for example DVDs) to be made with little or no loss of copy quality. This makes it easy for an unauthorised person to produce unauthorised copies of digital content carrying recording media. Accordingly, copy-protection techniques have been developed. Most current copy-protection techniques exploit differences in the ways a legitimate player and an unauthorised copying apparatus or “ripper” would access a recording medium in order to defeat copying while preserving good playability. For example, in some existing techniques subversive data is added to the digital data in such a manner that the subversive data is ignored by a legitimate player but corrupts the unauthorised copy made by a ripper, in other cases data is altered to affect the digital sum value (DSV) of some of the digital data in a manner which does not affect a legitimate player but interferes with an unauthorised copying apparatus or “ripper.
The above types of copy-protection may prevent digital copying of the content of an optical disc such as a DVD by a ripper that copies the data on a sector-by-sector or a file-by-file basis (for example “ISO/File mode rippers” for ripping DVDs). However, other more sophisticated rippers may be able to parse navigation information contained on the optical disc so as to select the content data be copied. Thus, for example, so-called “IFO parsing rippers” are capable of parsing navigation information contained on a DVD and automatically selecting the “main title” or the “main program chain (PGC)” for ripping or allowing the user the possibility of selecting a particular title or a particular program chain from amongst those available.